If the question is which of those two phrases to use in the home page's title, I can see that there are plenty of sites with home pages optimized for [horse directory], [horse services], or even [horse services directory]. Assuming your horse directory is full of lots of information concerning horses, don't worry right now about two specific, highly-competitive keyword phrases. As you build the directory, you're going to be creating lots of pages that are relevant to lots of searches -- the (ahem) long tail, if you will -- and the more of that you have, the more relevant the directory itself will become to the more general searches.